1. Field of the Invention
The present invention relates to a method for purifying an aqueous suspension with a module comprising wavy hollow fiber membranes. Particularly, the present invention relates to a method for purifying an aqueous suspension, for example, tap water such as river water, lake water, marsh water and groundwater; water for industrial use: waste water; secondary treatment waste water; industrial sewage; domestic sewage; human waste; sea water and the like with the module.
Further, the present invention is also directed to a bundle of wavy hollow fiber membranes, a process for producing the same, and a hollow fiber membrane module comprising the same.
2. Related Art
Heretofore, various methods for purifying aqueous suspensions such as tap water, water for industrial use, waste water, industrial sewage, domestic sewage, human waste, sea water and the like with a hollow fiber membrane have been known. In particular, a purifying method according to so-called external pressure filtration, in which raw water permeates in the direction from the outer to inner surface of a hollow fiber membrane, can secure a larger membrane area contributing to filtration per unit volume when compared to so-called internal pressure filtration, in which raw water permeates in the direction from the inner to outer surface of a hollow fiber membrane. Therefore external pressure filtration is advantageously used in the field wherein minimization of water production cost is required, for example, a water-treatment field such as turbidity removal for waterworks.
In the above method for purifying aqueous suspensions with hollow fiber membranes, suspended or organic materials having a size bigger than the pore diameter of the membranes, are hindered on the membrane surface to cause a so-called concentration polarization or form a cake layer. Moreover, the organic materials in aqueous suspension clog the pores of membranes or adhere to the inner network structure of membranes. As a result, the filtration flux upon filtering the raw water is decreased to about one tenth of that upon filtering pure water. Accordingly, there have been also known purifying methods in which physical wash of membranes is regularly practiced after the filtration in order to achieve a more stable filtration flux.
Specifically, it has been known to carry out, after a certain-term filtration, backward wash comprising feeding a part of filtrate in the reverse direction to the filtration, i.e., in the direction from the filtrate side to the raw water side (hereinafter simply referred to as back wash), air-scrubbing comprising supplying compressed gases and/or compressed air and raw water in the direction from the lower to upper part of the hollow fiber membrane module filled with water to shake fibers and discharge suspended solids accumulating among the hollow fiber membranes from the system, and the like. For example, Japanese Patent Application Laid-Open No. 60-19002 discloses a method in which a bubble generation nozzle is arranged on the side of or down the hollow fiber membranes in the hollow fiber membrane storage container, and a gas is injected therefrom together with back wash.
On the other hand, as a membrane which can be used for the above-mentioned purifying method, a reverse osmosis membrane, an ultrafiltration membrane, a microfiltration membrane, a gas separation membrane, a nanofilter, and a deairing membrane have been known. These membranes are not used alone but used in plural, i.e., in the form of a hollow fiber membrane module. The membrane module is prepared by mounting a plurality of the membranes in a module case, sealing at least one edge thereof with a thermosetting resin such as an epoxy resin, and cutting the bonded and fixed portion to open a hollow portion. Such a module is used in various fluid treatment fields, for instance, a reverse osmosis membrane module is used for desalination of sea water or brine, production of primary pure water of ultra-pure water, and concentration of fruit juice or milk; an ultrafiltration membrane module for collection of electrodeposition paints, production of pyrogen-free water, treatment of waste water, concentration of enzymes, final filtration of ultra-pure water, and turbidity removal from tap water or waste water; a microfiltration membrane module for turbidity removal from tap water or waste water, treatment of concentrated water, germ removal and purification of fermentation liquid, and fine particle removal from chemicals; a gas separation membrane module for steam removal, condensation of hydrogen, condensation or enrichment of oxygen, condensation or enrichment of nitrogen, and condensation of carbon dioxide; a nanofilter module for removal of agricultural chemicals or halogenated organic compounds; and a deairing membrane module for deairing of water and aqueous solution. The hollow fiber membranes per se have also been studied. For example, Japanese Patent Application Laid-Open No.64-22308 discloses the art using an external pressure filtration type hollow fiber membrane module wherein hollow fibers having wavy or spiral curls at least in a part thereof are mounted instead of the conventional straight hollow fiber membrane in order to prevent such a mutual clinging of the hollow fibers that hinders raw water from flowing toward the center of the module and to use almost all the hollow fibers mounted in the module for effective filtration.